Limit concurrency on GitHub Actions

Author: messense

.github/workflows/ci.yml

@@ -5,6 +5,10 @@ on:
       - main
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref_name }}-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: true
+
 jobs:
   fmt:
     runs-on: "ubuntu-latest"